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Glucose stimulation of islet beta-cell insulin secretion is initiated by membrane depolarization and an elevation in intracellular free calcium concentration (Ca2+i) from a combination of influx through depolarization-activated Ca2+ channels and intracellular Ca2+ store release. Prevention of Ca2+ store refilling with thapsigargin produced a sustained depolarization, leading to enhanced Ca2+ influx and an elevation in Ca2+i in 12 mM glucose. Depletion of intracellular Ca2+ stores by external EGTA reduced Ca2+i and also caused a long-lasting depolarization. In single beta-cells, external EGTA activated an inward current, the voltage range and kinetic properties of which differed from those of voltage-dependent Ca2+ channels. A novel pathway thus exists in beta-cells by which depletion of endoplasmic reticulum Ca2+ stores results in the activation of an inward current that, by inducing depolarization, facilitates Ca2+ influx through voltage-gated Ca2+ channels. The physiological relevance of this pathway in the control of beta-cell function is indicated by the stimulation of insulin secretion by thapsigargin.
Worley et al. (Sun,) studied this question.